When a high frequency pulse current or sinusoidal current is applied to an amorphous CoFeSiB based alloy wire, impedance changes with a magnetic field due to skin effect. This is a known phenomenon called magnetoimpedance effect (hereinafter abbreviated as MI effect). Some magnetoimpedance elements (hereinafter abbreviated as MI elements) directly detect this change by measuring impedance of the amorphous wire between ends thereof, and other MI elements detect this change by way of a detection coil wound around the amorphous wire. High-sensitive magnetic sensors using these MI elements are MI sensors.
These MI sensors are used in cellular phones and the like these days, but have a problem in that when sensor sensitivity is enhanced, measurement range is decreased. Conventionally control of sensitivity and measurement range has been carried out by two methods: a method using a demagnetizing field and a method controlling magnetic characteristics of a magnetosensitive wire. The method using a demagnetizing field is to reduce a longitudinal demagnetizing field by increasing length of a magnetosensitive wire in order to enhance sensitivity. However, because of the reduction in the demagnetizing field, measurement range is narrowed. Conversely, if length of a magnetosensitive wire is decreased, the longitudinal demagnetizing field is increased and measurement range is widened, but sensitivity is decreased. On the other hand, the method controlling magnetic characteristics of a magnetosensitive wire is to enhance sensor sensitivity by increasing magnetic permeability of the magnetosensitive wire in a longitudinal direction. However, due to this increase, measurement range of the magnetosensitive wire comprising a soft magnetic material which exhibits a magnetic saturation phenomenon is inevitably decreased. Conversely, if magnetic permeability in the longitudinal direction is decreased, measurement range is widened but naturally sensitivity is decreased. That is to say, sensitivity enhancement and measurement range increase are conflicting phenomena and are not compatible.
For example, as described in the official gazette of Japanese patent No. 3,693,119, when a pulse current having a frequency of 0.2 GHz, which was calculated by converting pulse rise time or fall time to frequency, was applied to a magnetosensitive wire having a wire diameter of 30 μm and a rather long length of 1.5 mm, a conventional MI sensor using this wire exhibited a sensitivity of 35 mV/G and a measurement range of 0.9 kA/m. This is a case of attaining high sensitivity using a demagnetizing field. On the other hand, when such a current was applied to a magnetosensitive wire having a wire diameter of 30 μm and a rather short length of 0.6 mm, a conventional MI sensor using this wire exhibited a sensitivity of 2 mV/G and a measurement range of 3.6 kA/m. This is a case of attaining wide measurement range using a demagnetizing field. Sensitivity and measurement range of a MI sensor have such a conflicting relationship as mentioned above, and therefore, it is difficult to improve the both simultaneously and there is a limit in practical use.
Here, an attempt to enhance sensitivity has been made by further increasing frequency of a high frequency electric current. L. V. Panina et al., Journal of Magnetism and Magnetic Materials, volumes 272-276 (2004), pp. 1452-1459 discloses measurement results of impedance of an amorphous wire between ends thereof when a sinusoidal current of 0.5 to 2.2 GHz was applied to the amorphous wire. According to the results, sensitivity was improved by increasing frequency but the problem is that measurement range was as remarkably low as 0.0125 A/m (10e) and was not widened by increasing frequency. Simultaneous improvement of both sensitivity and measurement range was not achieved.